While not limited thereto the present invention is particularly applicable to gas turbine engines such as used for the propulsion of aircraft.
Several types of gas turbine engines are currently available for powering aircraft. The turbofan and the turboprop are two examples of such engines. The turbofan engine includes a core engine, i.e., gas generator, for generating combustion gases which are expanded through a power turbine to drive a fan, whereas the turboprop engine includes a gas generator and power turbine which drives a propeller.
Conventional turboprop engines differ from turbofan engines in several fundamental respects. For example, turboprop engines typically have a much greater blade diameter than turbofan engines. This allows the blades to move a relatively large mass of air for producing thrust. Furthermore, for a given energy input to the blades, a relatively small velocity increase will be imparted to the air passing therethrough. Small velocity increases translate to high engine propulsive efficiencies. Simply stated, propulsive efficiency is a measure of how much available energy is converted to propulsive force. Large velocity increases to air passing through propulsor blades result in "wasted" kinetic energy and lower propulsive efficiency.
Turbofan engines move a somewhat smaller mass of air than do turboprops for the same energy input and impart a larger velocity component to the air in order to achieve the required thrust. This results in a lower propulsive efficiency. Turbofan engines also include a nacelle radially surrounding the fans. This creates an additional drag on the engine which degrades overall engine efficiency. However, the nacelle defines an inlet which diffuses the airstream entering the fan thereby slowing its speed. In this manner, air enters the fan with a relatively low axial velocity which is generally independent of flight speed. Such low axial velocities decrease blade drag losses thereby making higher cruise speeds attainable.
Intermediate-sized transport aircraft, for example, 100 to 180 passenger transports, typically utilize turbofan engines for propulsion. Turbofans provide the relatively high thrust required for powering these aircraft at relatively high altitudes and at cruise speeds of about Mach 0.6 to about Mach 0.8. For aircraft designed for lower cruise speeds, conventional turboprops are typically used inasmuch as they can provide superior performance and efficiency. For example, significant reductions in fuel burn, i.e., the amount of fuel consumed per passenger mile, are possible from the use of the aerodynamically more efficient turboprop over the turbofan.
Accordingly, it would be desirable to combine the advantages of the turbofan and the turboprop for obtaining a compound engine having improved overall engine efficiency at aircraft cruise speeds typical of turbofan powered aircraft.
The overall efficiency of an aircraft gas turbine engine is the product of thermal efficiency, transfer efficiency, and propulsive efficiency. Thermal efficiency is related to the core engine and is a measure of how effectively the energy in the fuel is converted to available energy in the core engine exhaust gases. Transfer efficiency is related to the structural engine components excluding the core engine and is a measure of how effectively core engine exhaust gas energy is converted into kinetic energy imparted to the air stream. Engine components which impact transfer efficiency include the propulsor blades, gearbox, power turbine, and engine nacelle. Accordingly, it is desirable to obtain a compound engine having relatively high transfer and propulsive efficiencies at relatively high subsonic Mach numbers.
A simple scaled up version of a conventional turboprop engine suitable for powering an intermediate-sized transport aircraft at the cruise speeds and altitudes typical of turbofan powered aircraft would require a single propeller of about 16 feet in diameter. It would also require the capability of generating about 15,000 shaft horsepower, which is several times the power output of conventional turboprop engines.
A conventional turboprop engine built to these requirements would further require the development of a relatively large and undesirably heavy reduction gearbox for transmitting the required power and torque at relatively low speed to the propeller. Such gearboxes tend to introduce losses which reduce the engine transfer efficiency. The rotational speed of the large diameter propeller is a limiting factor for keeping the helical velocity of the propeller tip, i.e., aircraft velocity plus tangential velocity of the propeller tip, below supersonic speeds. This is desirable inasmuch as a propeller tip operating at supersonic speeds generates a significant amount of undersirable noise and results in a loss of aerodynamic efficiency.
Gas turbine engines effective for driving propellers or fans without the use of a reduction gearbox are known in the prior art. They typically include relatively low speed, counterrotating turbine rotors having relatively few blade row stages driving a pair of counterrotating fans or propellers. These engines comprise various embodiments that utilize the fans or propellers for merely augmenting the thrust generated from the exhaust jet.
Such augmentation may be effective for some purposes. However, thrust augmentation requires that significant thrust is being produced by the exhaust gases exiting the power turbine and core nozzle. This reduces overall engine efficiency by degrading propulsive efficiency.
For propelling a modern, intermediate-sized aircraft that requires relatively large power output, a practical and relatively fuel efficient new generation engine having significant performance increases over conventional turbofan and turboprop engines and these counterrotating turbine rotor engines is required.
Accordingly, one object of the present invention is to provide a new and improved gas turbine engine.
Another object of the present invention is to provide a new and improved gas turbine engine for powering an aircraft at cruise speeds in excess of Mach 0.6 and less than 1.0 with improved overall engine efficiency.
Another object of the present invention is to provide a new and improved gas turbine engine including a power turbine having counterrotating rotors.
Another object of the present invention is to provide a new and improved gas turbine engine including a power turbine having a plurality of counterrotating turbine blade row stages wherein substantially all output power is obtained from expanding combustion gases through the stages.
Another object of the present invention is to provide a new and improved gas turbine engine wherein output power is obtainable without the use of a reduction gearbox.
Another object of the present invention is to provide a new and improved gas turbine engine effective for powering counterrotating airfoil members such as propellers.